Cedar River Project at Rainbow Bend
Twelve hundred feet of levee and bank protection along the right bank of the Cedar River downstream of Cedar Grove Road Bridge was removed to allow the river to expand and reconnect with 40 acres of the floodplain. Two pilot channels were excavated into the floodplain to help direct potentially damaging flood waters away from State Route 169 and the Cedar River Trail. Following construction, the site was replanted with native trees and shrubs.
Reconnecting the river to its floodplain will provide storage and conveyance of flood waters and will reduce the risk of damage along the river’s left bank, which guards State Route 169 and the Cedar River Trail. Removing the levee will also allow the Cedar River to freely access approximately 40 acres of floodplain, which will provide benefit to spawning and rearing salmon through the restoration of natural river processes.
The character of the reach will change and recreational users will need to take appropriate precautions.
With the levee removed, the channel downstream of the Cedar Grove Road Bridge will change. Expect to see the existing channel widen and shift; eroding the existing right (north) bank, creating new flow paths through the floodplain and capturing gravel and wood. The resulting channel will be more complex and dynamic; contain new obstacles such as downed trees and gravel bars that shift and adjust in response to high flow events. The speed of these changes will depend on weather, but it is possible for a substantial shift in channel location and shape to occur as early as this winter.
King County will monitor conditions in the reach and take action as appropriate in consultation with the King County Sheriff – Marine/Dive Unit. If you are a river user, please scout the reach before floating to identify potential new hazards and gage your ability to handle the evolving channel. To report hazardous river conditions, please contact the King County Sheriff - Marine/Dive Unit.
Phase 1 - Property Acquisition 2003-2012
Phase 2 – Site Preparation (Demolition, utility removal) 2011-2012
Phase 3 – Planning, Design, Construction 2010-2014
Monitoring and Maintenance 2015-2019
Adaptive Management and Long term Site Stewardship 2014 and beyond
- 30 percent project design plans (2.9 MB, PDF)
- 60 percent project design plans (10 MB, PDF)
- Final project design plans (11 MB, PDF)
Large wood checklists
The King County Flood Control District implemented a three-phase, high priority flood hazard management project in the Rainbow Bend reach of the Cedar River. The project will eliminate the flood risks to over 50 families and provide long term protection to a major regional transportation corridor. The project is also intended to improve habitat for fish and wildlife by allowing the river to widen, create side channels and slow water areas where fish can take refuge during high flow events.
Rainbow Bend was originally developed as a small neighborhood with 12 single family homes and a 50-unit mobile home park. It is located entirely in the 100-year floodplain and has experienced severe and repeated flooding.
The Rainbow Bend levee was neither intended nor built to provide 100-year flood protection for this area, and it is subject to frequent overtopping and backwater flooding, requiring regular damage repairs. During significant flood events, flood flows move fast and deep through the neighborhood and access to and from homes is cut off, threatening residents' safety and personal property.
Flows deflected by the obsolete Rainbow Bend levee had concentrated against the opposite bank, increasing velocities and flood elevations and threatening the Cedar River Trail Levee and the significant public infrastructure behind it, including:
- The primary transportation arterial for the Renton to Maple Valley corridor;
- a fiber optic cable line that runs from Seattle to Salt Lake City; and
- the regional trail that extends from the boundary of the upper watershed to Renton.
State Route 169 is used by 51,000 vehicles per day, according to the Washington State Department of Transportation. Failure of the Cedar Trail Levee would threaten these important elements of our public infrastructure and could force closure of the highway corridor.
This project will remove the levee, allowing floodwaters to pass safely across the floodplain, and reduce velocities by about 30 percent for a 100-year flood event, greatly reducing the threat to the highway and trail.
About Phases 1 & 2
In the first two project phases, from 2004 to 2011, King County worked with neighborhood residents to relocate them out of harm’s way and remove all structures from flood-prone lands. The first phase of the project involved acquisition and removal of the single family homes located immediately behind the levee. The second phase involved acquisition of the mobile home park and relocation of its residents.
For questions about this project, please contact Jon Hansen, Supervising Environmental Scientist, Ecological Restoration and Engineering Services.